High-frequency variability of small-particle carbon export flux in the Northeast Atlantic

Through the biological carbon pump, atmospheric CO2 is _xated by primary producers in the ocean and stored in the ocean interior, mostly in the form of sinking particles. These particle fluxes to the deeper layers of the ocean are a key factor in long-term carbon sequestration, but current estimates of global carbon export remain uncertain. An additional flux of carbon to the mesopelagic zone can be supplied by a process known as the mixed-layer pump: a seasonal net detrainment of surface waters, caused by sequential shoaling and deepening of the mixed layer in spring. In this study, we present a full year of daily particulate organic carbon (POC) concentrations derived from glider optical backscatter data, at the Porcupine Abyssal Plain (PAP) sustained observatory in the Northeast Atlantic. We observe a strong seasonality in remineralisation depth and transfer efficiency, closely related to variations in mixed-layer depth. By creating a model of daily POC export by mixed-layer variations, we find that the seasonal mixedlayer pump supplies a flux of 6.3 g POC m-2 yr-1 to the mesopelagic zone. In the PAP site, this pump is driven by a deep mixed layer of max. 350m deep, with frequent daily mixed-layer depth variations of up to 90m. Moreover, by extending the period of the seasonal mixed-layer pump to an annual scale, we observe an additional flux of 1.0 g POC m-2 yr-1 to the mesopelagic. Comparing our results to various model estimates of carbon export in the PAP site, the seasonal mixed-layer pump contributes between 13 and 43% of the total export. Comparison of glider data with remotely sensed wind speed and net surface heat flux suggests that shoaling of the mixed layer in spring is an important control of carbon export, driven by the balance between wind and convective mixing regimes. This is, to our best knowledge, the first high-frequency estimate capturing export variability over the course of a full year. Our results support the deployment of bio-optical sensors on gliders to improve our understanding of the ocean carbon cycle on various temporal scales.